1. Field of the Invention
The present invention relates to a magnetic bearing device and a pump device with the magnetic bearing device mounted thereto. More specifically, the present invention relates to a magnetic bearing device capable of reducing vibration, as well as electric and magnetic noise, generated from a pump device while making it possible to control the position of a rotor at high speed and high power, and to a pump device having the magnetic bearing device mounted thereto.
2. Description of the Related Art
With the development of electronics in recent years, demands for semiconductors for forming memories, integrated circuits, etc. are rapidly increasing.
Those semiconductors are manufactured such that impurities are doped into a semiconductor substrate with a very high purity to impart electrical properties thereto, or semiconductor substrates with minute circuit patterns formed thereon are laminated.
Those manufacturing steps must be performed in a chamber with a high vacuum state so as to avoid influences of dust etc. in the air. A vacuum pump is generally used as a pump device to evacuate the chamber. In particular, a turbo molecular pump, which is one kind of the vacuum pumps, is widely used since it entails little residual gas and is easy of maintenance and so on.
The semiconductor manufacturing process includes a number of steps in which various process gases are caused to act onto a semiconductor substrate, and the turbo molecular pump is used not only to evacuate the chamber but also to discharge those process gases from the chamber.
Further, in equipment for an electron microscope etc., a turbo molecular pump is used to create a high vacuum state in the chamber of the electron microscope etc. in order to prevent refraction etc. of an electron beam caused by the presence of dust or the like.
FIG. 7 is a longitudinal sectional view of the turbo molecular pump.
In FIG. 7, a turbo molecular pump 100 includes an outer cylinder 127 with an intake hole 101 formed on top thereof. Provided inside the outer cylinder 127 is a rotor 103 having in its periphery a plurality of rotor blades 102a, 102b, 102c, . . . serving as turbine blades for sucking and discharging gas and formed radially in a number of stages.
At the center of the rotor 103, a rotor shaft 113 is mounted with being supported in a levitating state in the air and controlled in position, for example, by a 5-axis control magnetic bearing.
Upper radial electromagnets 104 include four electromagnets arranged in pairs in X- and Y-axis directions. Further, there is provided an upper radial sensor 107 constituted of four electromagnets arranged in close proximity to and in correspondence with the upper radial electromagnets 104. The upper radial sensor 107 detects radial displacement of the rotor 103, transmitting a detection signal to a control device (not shown in the drawing).
In this control device, excitation of the upper radial electromagnets 104 is controlled by an amplifier circuit 150 (will be discussed later) through a compensation circuit having a PID adjusting function, on the basis of a displacement signal detected by the upper radial sensor 107, thus performing adjustment of the radial position of the upper portion of the rotor shaft 113.
The rotor shaft 113 is formed of a high-magnetic-permeability material (e.g., iron) and is adapted to be attracted by the magnetic force of the upper radial electromagnets 104. Such adjustment is conducted independently in the X-axis direction and the Y-axis direction.
Further, lower radial electromagnets 105 and a lower radial sensor 108 are arranged in the same way as the upper radial electromagnets 104 and the upper radial sensor 107. Like the radial position of the upper portion of the rotor shaft 113, the radial position of the lower portion of the rotor shaft 113 is adjusted.
Further, axial electromagnets 106A and 106B each are arranged on the upper and lower sides of a metal disc 111 provided in the lower portion of the rotor shaft 113. The metal disc 111 is formed of a high-magnetic-permeability material such as iron. To detect axial displacement of the rotor shaft 113, an axial sensor 109 is provided, which transmits an axial displacement signal to the control device.
The axial electromagnets 106A and 106B are excitation-controlled by the amplifier circuit 150 through the compensation circuit, which has a PID adjusting function, of the control device on the basis of the axial displacement signal. The axial electromagnet 106A upwardly attracts the magnetic disc 111 by the magnetic force, and the axial electromagnet 106B downwardly attracts the magnetic disc 111.
In this way, the control device has a function to appropriately control the magnetic force exerted on the metal disc 111 by the axial electromagnets 106A and 106B to magnetically levitate the rotor shaft 113 in the axial direction, thereby retaining the rotor shaft 113 in the space in a non-contact state.
Note that descriptions will be given later on the amplifier circuit 150 that drives, through excitation, the upper radial electromagnets 104, the lower radial electromagnets 105, and the axial electromagnets 106A and 106B.
A motor 121 is equipped with a plurality of magnetic poles, which are arranged circumferentially to surround the rotor shaft 113. The magnetic poles are controlled by the control device to rotate the rotor shaft 113 through an electromagnetic force acting between the rotor shaft 113 and the magnetic poles.
The motor 121 also has an RPM sensor (not shown in the drawing) incorporated to output a detection signal, which is used for detection of RPM of the rotor shaft 113.
A phase sensor (not shown in the drawing) is attached, for example, in the vicinity of the lower radial sensor 108 to detect the phase of rotation of the rotor shaft 113. From detection signals of the phase sensor and the RPM sensor both, the control device detects positions of the magnetic poles.
A plurality of stationary blades 123a, 123b, 123c, . . . are arranged so as to be spaced apart from the rotor blades 102a, 102b, 102c, . . . by small gaps. To downwardly transfer the molecules of exhaust gas through collision, the rotor blades 102a, 102b, 102c, . . . are inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113.
Similarly, the stationary blades 123 are also inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113, and extend toward the inner side of the outer cylinder 127 to be arranged alternately with the rotor blades 102.
The stationary blades 123 are supported at one end by being inserted into gaps between a plurality of stationary blade spacers 125a, 125b, 125c, . . . stacked together in stages.
The stationary blade spacers 125 are ring-shaped members, which are formed of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing such metal as a component.
In the outer periphery of the stationary blade spacers 125, the outer cylinder 127 is secured in position with a small gap therebetween. At the bottom of the outer cylinder 127, there is arranged a base portion 129, and a threaded spacer 131 is arranged between the lowermost one of the stationary blade spacers 125 and the base portion 129. In the portion of the base portion 129 below the threaded spacer 131, there is formed a discharge outlet 133 which communicates with the outside.
The threaded spacer 131 is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or an alloy containing such metal as a component, and has a plurality of spiral thread grooves 131a in its inner peripheral surface.
The spiral direction of the thread grooves 131a is determined such that when the molecules of the exhaust gas move in the rotating direction of the rotor 103, these molecules are transferred toward the discharge outlet 133.
Connected to the lowermost one of the rotor blades 102a, 102b, 102c, . . . of the rotor 103 is a rotor blade 102d, which extends vertically downwards. The outer peripheral surface of the rotor blade 102d sticks out toward the inner peripheral surface of the threaded spacer 131 in a cylindrical shape, and is in close proximity to the inner peripheral surface of the threaded spacer 131 with a predetermined gap therebetween.
The base portion 129 is a disc-like member constituting the base of the turbo molecular pump 100, and is generally formed of a metal, such as iron, aluminum, or stainless steel.
The base portion 129 physically retains the turbo molecular pump 100, and also functions as a heat conduction passage. Thus, the base portion 129 is preferably formed of a metal that is rigid and of high heat conductivity, such as iron, aluminum, or copper.
In the above-described construction, when the rotor blades 102 are driven and rotated by the motor 121 together with the rotor shaft 113, an exhaust gas from a chamber is sucked in through the intake hole 101 by the action of the rotor blades 102 and the stationary blades 123.
The exhaust gas sucked in through the intake hole 101 passes between the rotor blades 102 and the stationary blades 123, and is transferred to the base portion 129. At this point, the temperature of the rotor blades 102 is raised by frictional heat generated as the exhaust gas comes into contact with the rotor blades 102 and by heat generated and conducted from the motor 121. Such heat is transferred to the stationary blades 123 through radiation or through conduction of gas molecules of exhaust gas or the like.
The stationary blade spacers 125 are joined to one another on the outer periphery and send, to the outside, heat which the stationary blades 123 receive from the rotor blades 102 as well as frictional heat generated upon contact between exhaust gas and the stationary blades 123.
The exhaust gas transferred to the base portion 129 is sent to the discharge outlet 133 while being guided by the thread grooves 131a of the threaded spacer 131.
In the description above, the threaded spacer 131 is placed on the outer periphery of the rotor blade 102d and the inner peripheral surface of the threaded spacer 131 is scored with the thread grooves 131a. This may be reversed and the outer peripheral surface of the rotor blade 102d may be scored with thread grooves, whereas a spacer of which inner peripheral surface forms a cylindrical shape surrounds the rotor blade 102d. 
Further, in order to prevent the exhaust gas sucked in through the intake hole 101 from entering the electrical portion composed of the motor 121, the lower radial electromagnet 105, the lower radial sensor 108, the upper radial electromagnet 104, the upper radial sensor 107, etc., the electrical portion is covered with a stator column 122, and the interior of this electrical portion is maintained at a predetermined pressure with a purge gas.
For this purpose, the base portion 129 is equipped with piping (not shown in the drawing), and the purge gas is introduced through the piping. The purge gas introduced is passed through the gap between a protective bearing 120 and the rotor shaft 113, the gap between the rotor and stator of the motor 121, and the gap between the stator column 122 and the rotor blades 102 before it is transmitted to the discharge outlet 133.
The turbo molecular pump 100 requires control based on individually adjusted specific parameters (e.g., identification of the model and characteristics corresponding to the model). To store the control parameters, the turbo molecular pump 100 contains an electronic circuit portion 141 in its main body. The electronic circuit portion 141 is composed of a semiconductor memory, such as EEP-ROM, electronic parts, such as semiconductor devices for access to the semiconductor memory, a substrate 143 for mounting these components thereto, etc.
This electronic circuit portion 141 is accommodated under an RPM sensor (not shown in the drawing) near the center of the base portion 129 constituting the lower portion of the turbo molecular pump 100, and is closed by a hermetic bottom cover 145.
In some cases, a process gas is introduced to a chamber with its temperature raised in order to enhance the reactivity. Such process gas is cooled upon discharge and, reaching a certain temperature, could change into a solid to precipitate in the exhaust system. This type of process gas, one that becomes solid when cooled, adheres to the interior of the turbo molecular pump 100 and builds up.
For instance, a vapor pressure curve shows that SiCl4 used as a process gas for an Al etching device precipitates at low vacuum (760 torr to 10−2 torr) and low temperature (about 20° C.) to produce a solid product (e.g., AlCl3), which adheres and builds up in the turbo molecular pump 100. As the precipitate of the process gas builds up in the turbo molecular pump 100, the pump flow path is clogged with the deposit, thereby lowering the performance of the turbo molecular pump 100. The solid product tends to coagulate and adhere in the area near the discharge outlet where the temperature is low, in particular, around the rotor blades 102 and the threaded spacer 131.
A conventional measure taken to solve this problem is to wind a heater (not shown in the drawing) and a ring-like water-cooled tube 149 around the outer periphery of the base portion 129 or other portion while burying a temperature sensor (not shown in the drawing) (e.g., thermistor) in, for example, the base portion 129, so that the temperature of the base portion 129 is kept high at a set temperature by controlling the heating effect of the heater and the cooling effect of the water-cooled tube 149 based on a signal from the temperature sensor (temperature management system, hereinafter abbreviated as TMS).
Given next is a detailed description of the amplifier circuit 150, which drives, through excitation, the upper radial electromagnets 104, the lower radial electromagnets 105, and the axial electromagnets 106A and 106B of the turbo molecular pump 100 structured as above.
A circuit diagram of this amplifier circuit is shown in FIG. 8.
In FIG. 8, the electromagnet coil 151, which constitutes the upper radial electromagnets 104 or other electromagnets, is connected at one end to a positive electrode 171a of a power source 171 through a transistor 161 and is connected at the other end to a negative electrode 171b of the power source 171 through a current detecting circuit 181 and through a transistor 162.
The transistors 161 and 162 are so-called power MOSFETs, which are structured to have diodes connected between their sources and drains.
A cathode terminal 161a of a diode of the transistor 161 is connected to the positive electrode 171a, whereas an anode terminal 161b of the diode is connected to one end of the electromagnet coil 151.
A cathode terminal 162a of a diode of the transistor 162 is connected to the current detecting circuit 181, whereas an anode terminal 162b of the diode is connected to the negative electrode 171b. 
On the other hand, a diode 165 provided for current regeneration is connected at its cathode terminal 165a to one end of the electromagnet coil 151 and is connected at its anode terminal 165b to the negative electrode 171b. 
Similarly, a diode 166 for current regeneration has a cathode terminal 166a connected to the positive electrode 171a and has an anode terminal 166b connected to the other end of the electromagnet coil 151 through the current detecting circuit 181.
The current detecting circuit 181 is a hole sensor serving as a current sensor, for example.
The amplifier circuit 150 structured as above is for the electromagnets 104, 105, 106A, and 106B, whereas another amplifier circuit 150 is built for other electromagnets 105, 106A, and 106B. Accordingly, in the case of a 5-axis control magnetic bearing, there are ten of the same amplifier circuits (each of which is denoted by 150) connected in parallel to the power source 171.
An amplifier control circuit 191 is a circuit in a digital signal processor unit (not shown in the drawing) (hereinafter abbreviated as DSP unit) of the control device. The amplifier control circuit 191 makes it possible to switch on and off the transistors 161 and 162.
To elaborate, the DSP unit and its amplifier control circuit 191 compare a current value detected by the current detecting circuit 181 (a signal that reflects this current value is called a current detection signal 191c) with a predetermined current command value. Based on the result of the comparison, the pulse width (pulse width time Tp1 or Tp2) of pulses to be generated in a control cycle Ts, which is one cycle by PWM control, is determined. Then the amplifier control circuit 191 outputs gate drive signals 191a and 191b having the thus determined pulse width to gate terminals of the transistors 161 and 162.
The power source 171 has to control the position of the rotor 103 at high speed and high power upon passing a resonance point while the rotor 103 is operated at an accelerated rotation speed, or upon encountering disturbance during rotor's constant speed operation, or like other cases. For that reason, the power source 171 has to be capable of handling a rapid increase (or decrease) in current flowing in the electromagnet coil 151 and accordingly is a high voltage power source of about 50 V, for example.
Usually, a capacitor (omitted from the drawing) is connected between the positive electrode 171a and the negative electrode 171b of the power source 171 in order to stabilize the power source 171.
In this structure, the transistors 161 and 162 are both turned on to increase a current flowing in the electromagnet coil 151 (hereinafter referred to as electromagnet current iL) and the electromagnet current iL is reduced by turning both of the two transistors off.
When only one of the transistors 161 and 162 is turned on, a so-called flywheel current is maintained. By letting a flywheel current flow in the amplifier circuit 150, the hysteresis loss in the amplifier circuit 150 can be reduced and the power consumption of the circuit in total can be lowered as disclosed in JP 3176584 B. In addition, with the transistors 161 and 162 controlled in this manner, high-frequency noise of harmonics or the like is reduced in the turbo molecular pump 100. Moreover, the electromagnet current iL flowing in the electromagnet coil 151 can be detected by measuring the flywheel current with the current detecting circuit 181.
Specifically, in the case where the current command value is larger than a current value detected, the transistors 161 and 162 are both turned on once in the control cycle Ts (for example, 100 μs) for a time period that corresponds to the pulse width time Tp1 as shown in FIG. 9. The electromagnet current iL during this period is thus increased toward a current value iLmax (not shown in the drawing), which is a maximum current possible to flow from the positive electrode 171a to the negative electrode 171b through the transistors 161 and 162.
On the other hand, in the case where the command value is smaller than the detection value, the transistors 161 and 162 are both turned off once in the control cycle Ts for a time period that corresponds to the pulse width time Tp2 as shown in FIG. 10. The electromagnet current iL during this period is thus decreased toward a current value iLmin (not shown in the drawing) that can be regenerated from the negative electrode 171b to the positive electrode 171a through the diodes 165 and 166.
In either case, one of the transistors 161 and 162 is turned on after the elapse of the pulse width time Tp1 or Tp2. Thus a flywheel current is maintained in the amplifier circuit 150 during this period.
As described above, it is necessary for the turbo molecular pump 100 to place the upper radial electromagnets 104 and the upper radial sensor 107 as close to each other as possible.
This also applies to arrangement of other electromagnets and sensors, and the distance between the lower radial electromagnets 105 and the lower radial sensor 108 and the distance between the axial electromagnets 106A and 106B and the axial sensor 109 have to be as short as possible.
Accordingly, it can be said that the electromagnets 104, 105, and 106A and 106B are electrostatically coupled to their respective position sensors 107, 108, and 109.
The rotor shaft 113 and the metal disc 111, which are formed from high-magnetic-permeability materials, are interposed between the electromagnets 104, 105, and 106A and 106B and their respective position sensors 107, 108, and 109, and are magnetically coupled to the electromagnets and to the sensors.
The amplifier control circuit 191 switches on and off the transistors 161 and 162 at high speed within a period equal to or shorter than the control cycle Ts as described above. This could cause electric noise (for example, noise due to an inductance component and noise due to reflection) in the electromagnet coil 151 upon switching on or off the transistors 161 and 162. In addition, electric noise in the electromagnet coil 151 sometimes induces magnetic noise in the electromagnet coil 151.
Electric and magnetic noise generated in the electromagnetic coil 151 can influence the position sensors 107, 108, and 109 coupled to the electromagnet coil 151 electrostatically and magnetically.
If electric noise generated in the electromagnet coil 151 affects the position sensors 107, 108, and 109, noise in an amount corresponding to the noise component could be mixed in with displacement signals detected by the position sensors 107, 108, and 109. A displacement signal with a noise component mixed in is used in PID control by the compensation circuit of the control circuit, which outputs the result of the PID control to the amplifier circuit 150. Consequently, excitation-drive of the electromagnet coil 151 by the amplifier circuit 150 is affected by the noise component mixed in, causing the rotor 103 to vibrate in accordance with an electric noise component that is not the true displacement signal.
Similarly, if magnetic noise generated in the electromagnet coil 151 affects the position sensors 107, 108, and 109, it could cause noise in magnetic fields detected by the position sensors 107, 108, and 109. As in the case of electric noise described above, this allows noise in an amount corresponding to the noise component mixed in with displacement signals detected by the position sensors 107, 108, and 109. Consequently, the rotor 103 vibrates in accordance with a magnetic noise component that is not the true displacement signal.
With the rotor 103 vibrated by a noise component, the vibration is transferred to a chamber to which the turbo molecular pump 100 is connected and influences operations conducted in the chamber, including manufacture of a semiconductor and measurement using an electron microscope.
Moreover, being of electric and magnetic nature, noise components generated in the electromagnet coil 151 could be transferred directly to the chamber to influence semiconductor manufacture, electron microscope measurement, and other operations performed in the chamber. In measurement using an electron microscope, in particular, magnetic noise generated in the electromagnet coil 151 can bend the trajectory of an electron beam emitted in the electron microscope.
Furthermore, electric and magnetic noise in the electromagnet coil 151 is increased in proportion to the voltage level of the power source 171, which as has been described is a high voltage power source of about 50 V, usually, in order to control the position of the rotor 103 at high speed and high power. It is therefore difficult to reduce electric and magnetic noise in the electromagnet coil 151 while controlling the position of the rotor 103 at high speed and high power.